There are many methods available in the market for scrubbing flue gases to remove pollutant gases and common methods available in the market may be classified in the following few categories:—                1. Chemical neutralizing scrubbing methods, either using alkali chemicals such as NaOH, MgO, CaO, or using seawater bicarbonates/carbonates to neutralize the acid gases.        2. Urea Selective Catalytic Reduction (SCR) methods, which reduce NOx.        3. Carbon Capture and Storage (CCS) methods, which bury or store CO2 (this is not a removal process).        4. DC electrolysis and time-varying electromagnetic wave treatments.        
Each of the above methods has certain deficiencies and either are impractical to implement or are unable to solve environmental problems completely, hence limiting their application as “Total Green Solutions”.
Chemical neutralizing scrubbing methods are used to remove SOx gases. However, they create secondary pollution problems such as acidifying the seawater, emission of CO2 in the process of producing the scrubbing chemicals such as CaO, and generation of more CO2 in the scrubbing of SOx when using seawater. Additionally, the disposal of the chemical reaction end products is always a major environmental issue.
SCR methods only remove NOx and are unable to remove SOx or CO2. Additionally, the catalyst of the SCR is easily poisoned by SO2 in the flue gases and this makes SCR difficult to implement in gases containing SO2.
CCS methods do not actually remove CO2. They specifically are storage methods and cannot be considered as pollutant removal processes. In practice, they may actually emit more CO2 in the storage process and they also require SOx to be removed first.
DC electrolysis and time-varying electromagnetic wave treatments cover a variety of different methods. Sukheon An and Osami Nishida (JSME International Journal Series B, Volume 46, Issue 1, pp. 206-213 (2003)) teach the use of DC electrolysis to remove SOx, CO2 and NOx gases by a chemical neutralization process using the chemicals produced in a DC electrolysis process. However, in this DC electrolysis process, the anode water becomes very acidic and causes severe corrosion and disposal issues.
WO2010/139114 (which describes open loop methods) and WO2011/147085 (which describes closed loop methods) describe the use of DC electrolysis and time-varying electromagnetic wave methods to remove SOx, CO2 and NOx.
As described in WO2010/139114, SO2 is removed by a chemical neutralizing process using pre-electrolyzed alkaline water. The removal of CO2 and NOx is by way of electrolyzing and the use of time-varying electromagnetic treated water to break the CO2 into C and O2 and NOx into N2 and O2.
In both WO2010/139114 and WO2011/147085, the gas removal function is performed by the DC electrolysis process which generates an electrolysis plasma in the water. The alkali-producing treatment effect in the DC electrolysis is also used to maintain the water pH in the respective alkaline stages. However, DC electrolysis has a major disadvantage in many applications in that when it takes place in fresh water or seawater, inevitably hydrogen gas and chlorine/hypochlorite will be generated. While chlorine/chlorite can be eliminated if magnesium is used as the anode material, hydrogen gas generation is inevitable. Additionally, if a magnesium electrode is used, it is consumable and a high cost is incurred. Hydrogen gas and chlorine gas generation are both hazardous due to their explosive and toxic properties which are highly undesirable for industrial applications especially in marine oil tankers, LNG carriers, refineries or other safety sensitive applications.
In WO2010/139114, the use of a time-varying electromagnetic treatment is based on the disclosure of PCT/SG2006/000218. In this disclosure, the time-varying electromagnetic wave is a pure AC wave and it uses an indirect inductor coil external field treatment method to treat the water. This time-varying electromagnetic wave treatment using a pure AC pulsed wave to energize the inductor coils or emitter is unable to control the pH. In practice, no perfect plasmatic gas phase reaction can take place when water is in contact with sour gas so inevitably a side liquid phase reaction will take place and the water will gradually become acidic, although at a slower rate.
WO2011/147085 has a first stage which operates at a very acidic condition of pH 2˜4, which is highly corrosive. This requires special alloy materials for the scrubbing towers, pumps, electrodes, associated piping materials and tanks etc. The teaching of WO2011/147085 is based on the principle of using a “fully saturated” solution to promote the gas-breaking reaction and preventing the SOx, CO2 and NOx from entering into the water medium. When a fully saturated solution is Used, precipitation of solutes takes place easily and they will choke up nozzles, pipes etc.
Another drawback of the disclosure of WO2011/147085 is that the breaking of SO2 into S and O2 creates another environmental issue due to the difficulty of storing and disposing of sulphur produced from the breaking process, especially for marine or shipboard applications.
Yet another drawback of the disclosure of WO2011/147085 is that the scrubbing of SOx, NOx and CO2 needs to be carried out in three different stages and these three stages are supposed to be separated and operate as three independent systems with their own specific scrubbing media and operating conditions. This is because the removal of SOx, NOx and CO2 as described in WO2011/147085 respectively requires a different set of reaction environment conditions and it is important that they should not be cross-contaminated. Once contaminated, the reaction environment is changed and gas removal performance is compromised. In actual practice, as the three stages described in WO2011/147085 share the same common gas path for the scrubbing processes, the media used in the three stages will be carried over and contamination will happen. Critically, when the first stage acidic pH medium is carried over to the alkaline environment of stages 2 and 3, the stage 2 and 3 pH condition is changed and the chemical composition of each buffer medium is changed too. This leads to a deterioration in performance and, in practice, it is difficult to eradicate contamination between three separate reaction environment sensitive systems if they are installed in one common vertical pass gas path axis, hence limiting application of the three-stage method in many industries.
More precisely, WO2011/147085 teaches that SOx, CO2 or NOx gas can only be removed one gas at a time in separate stages as it needs a specific critical reaction environment and condition to remove each particular gas. In the method however all three gases are removed in one processing tower. This comprises the three different stages and does not remove the gases simultaneously.
It has been always a challenge to develop alternative technologies for obtaining various treatment effects that are effective and cause no harm to the environment. Therefore, there is a need for new methods and systems that are capable of carrying out effective removal of pollutant gases from flue gases and that do not cause harm to the atmosphere or surrounding environments.
The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.